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Placodermi
Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally "plate-skinned") is an extinct class of armoured prehistoric fish, known from fossils, which lived from the Silurian to the end of the Devonian Period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked, depending on the species. Placoderms were among the first jawed fish; their jaws likely evolved from the first of their gill arches. Placoderms are paraphyletic, and comprise several distinct outgroups or sister taxa to all living jawed vertebrates, which originated among their ranks. This is illustrated by a 419-million-year-old fossil,Entelognathus, from China, which is the only known placoderm with a type of bony jaw like that found in modern bony fishes. This includes a dentary bone, which is found in humans and other tetrapods,.1 The jaws in other placoderms were simplified and consisted of a single bone.2 Placoderms were also the first fish to develop pelvic fins, the precursor to hindlimbs in tetrapods, as well as true teeth.3 380-million-year-old fossils of three other genera, Incistoscutum, Materpiscis and Austroptyctodus, represent the oldest known example of live birth.4 The first identifiable placoderms appear in the fossil record during the late Llandovery epoch of the early Silurian.5 The various groups of placoderms were diverse and abundant during the Devonian, but became extinct at the end-Devonian Hangenberg event 360 million years ago 6 Contents hide * 1Characteristics * 2Evolution and extinction ** 2.1Fossil record ** 2.2History of study * 3Taxonomy and phylogeny * 4Cladogram * 5See also * 6Notes * 7References * 8External links Characteristicsedit Many placoderms, particularly the Rhenanida, Petalichthyida, Phyllolepida, and Antiarchi, were bottom-dwellers. In particular, the antiarchs, with their highly modified, jointed bony pectoral fins, were highly successful inhabitants of Middle-Late Devonian freshwater and shallow marine habitats, with the Middle to Late Devonian genus,Bothriolepis, known from over 100 valid species.7 The vast majority of placoderms were predators, many of which lived at or near the substrate. Many, primarily theArthrodira, were active, nektonic predators that dwelled in the middle to upper portions of the water column. A study of the arthrodire Compagopiscis published in 2012 concluded that placoderms (at least this particular genus) likely possessed true teeth contrary to some early studies. The teeth had well defined pulp cavity and were made of both bone and dentine. However the tooth and jaw development were not as closely integrated as in modern gnathostomes. These teeth were likely homologous to the teeth of other gnathostomes.3 External anatomy of the placoderm Coccosteus decipiens One of the largest known arthrodires, Dunkleosteus terrelli , was 8 to 11 m (26–36 ft) long, and is presumed to have had a large distribution, as its remains have been found in Europe, and North America and possibly Morocco. Some paleontologists regard it as the world's first vertebrate "superpredator", preying upon other predators. Other, smaller arthrodires, such as''Fallacosteus'' and Rolfosteus, both of the Gogo Formation of Western Australia, had streamlined, bullet-shaped head armor, strongly supporting the idea that many, if not most, arthrodires were active swimmers, rather than passive ambush-hunters whose armor practically anchored them to the sea floor. Some placoderms were herbivorous, such as the Middle to Late Devonian arthrodire Holonema, and some were planktivores, such as the gigantic, 7 to 8 m (23–26 ft) long arthrodire, Titanichthys. Extraordinary evidence of internal fertilization in a placoderm was afforded by the discovery in the Gogo Formation, near Fitzroy Crossing, Kimberley, Western Australia,8 of a small female placoderm, about 25 cm (10 in) in length, which died in the process of giving birth to a 6 cm (2 1⁄2 in) live young one and was fossilized with the umbilical cord intact.9 The fossil, named Materpiscis attenboroughi (after scientist David Attenborough), had eggs which were fertilised internally, the mother providing nourishment to the embryo and giving birth to live young. With this discovery, the placoderm became the oldest vertebrate known to have given birth to live young ("viviparous"),4 pushing the date of first viviparity back some 200 million years earlier than had been previously known. Specimens of the arthrodire Incisoscutum ritchei, also from the Gogo Formation, have been found with embryos inside them indicating this group also had live bearing ability.10 The males reproduced by inserting a long clasper into the female. Long basipterygia are also found on the phyllolepid placoderms, such as Austrophyllolepis11 and Cowralepis, both from the Middle Devonian of Australia, suggesting that the basiptergia were used in copulation. The placoderm claspers' are not homologous with the claspers in cartilaginous fishes. The similarities between the structures has been revealed to be an example of convergent evolution. While the claspers in cartilaginous fishes are specialized parts of their paired pelvic fins that has been modified for copulation due to changes in the hox genes hoxd13, the origin of the mating organs in placoderms most likely relied on different sets of hox genes and were structures that developed further down the body as an extra and independent pair of appendages, but which during development turned into body parts used for reproduction only. Because they were not attached to the pelvic fins, as the claspers in fish like sharks, they were much more flexible and could probably be rotated forward.12 Evolution and extinctionedit Evolution and extinction of placoderms. The diagram is based on Michael Benton, 2005.13 Dunkleosteus, among the first of the vertebrate apex predators, was a giant armoured placoderm predator. See also: Evolution of fish It was thought for a time that placoderms went extinct due to competition from the first bony fish and early sharks, given a combination of the supposed inherent superiority of bony fish and the presumed sluggishness of placoderms. With more accurate summaries of prehistoric organisms it is now thought that they died out as marine and freshwater ecologies suffered the environmental catastrophes of the Devonian/Carboniferous extinction events Fossil recordedit The earliest identifiable placoderm fossils are from China and date to the early Silurian. At that time, they were already differentiated into antiarchs and arthrodires as well as other, more primitive groups. Earlier fossils of basal Placodermi have not yet been discovered. The Silurian fossil record of the placoderms is both literally and figuratively fragmented. With one exception, all known Silurian placoderms exist today only as fragments, either scraps of armor or isolated scales. Some of them have been tentatively identified as antiarch or arthrodire due to histological similarities; and many of them have not yet been formally described or even named. The most commonly cited example of a Silurian placoderm, Wangolepis of Silurian China and possibly Vietnam, is known only from a few fragments that currently defy attempts to place them in any of the recognized placoderm orders. So far, only two officially described Silurian placoderms known from more than scraps: * the basal antiarch Silurolepis, from the Ludlow epoch of Yunnan, China, known from an almost complete thoracic armor * Entelognathus, a placoderm incertae sedis that combines features of primitive arthrodires with jaw anatomy otherwise only seen in bony fish and tetrapods. The first officially described and oldest Silurian placoderm is an antiarch, Shimenolepis, which is known from distinctively ornamented plates from the late Llandovery ofHunan, China. Shimenolepis plates are very similar to the early Devonian yunnanolepid Zhanjilepis, also known from distinctively ornamented plates.514 Paleontologists and placoderm specialists suspect that the scarcity of placoderms in the Silurian fossil record is due to placoderms' living in environments unconducive to fossil preservation, rather than a genuine scarcity. This hypothesis helps to explain the placoderms' seemingly instantaneous appearance and diversity at the very beginning of the Devonian. During the Devonian, the placoderms went on to inhabit and dominate almost all known aquatic ecosystems, both freshwater and saltwater.15 But this diversity ultimately suffered many casualties during the extinction event at the Frasnian–Famennian boundary, the Late Devonian extinctions. The remaining species then died out during the Devonian/Carboniferous extinction event; not a single placoderm species has been confirmed to have survived into the Carboniferous. History of studyedit The earliest studies of placoderms were published by Louis Agassiz, in his five volumes on fossil fishes, 1833–1843. In those days, the placoderms were thought to be shelled jawless fish akin to ostracoderms. Some naturalists even suggested that they were shelled invertebrates or even turtle-like vertebrates. In the late 1920s, Dr. Erik Stensiö, at the Swedish Museum of Natural History in Stockholm, established the details of placoderm anatomy and identified them as true jawed fishes related to sharks. He took fossil specimens with well-preserved skulls and ground them away, one tenth of a millimeter at a time. Between each grinding, he made an imprint in wax. Once the specimens had been completely ground away (and so destroyed), he made enlarged, three-dimensional models of the skulls to examine the anatomical details more thoroughly. Many other placoderm specialists thought that Stensiö was trying to shoehorn placoderms into a relationship with sharks; however, as more fossils were found, placoderms were accepted as a sister group of chondrichthyians. Much later, the exquisitely preserved placoderm fossils from Gogo reef changed the picture again. They showed that placoderms shared anatomical features not only with chondrichthyians but with othergnathostome groups as well. For example, Gogo placoderms show separate bones for the nasal capsules as in gnathostomes; in both sharks and bony fish those bones are incorporated into the braincase.1617 Placoderms also share anatomical features only with the jawless osteostracans, so the theory that placoderms are the sister group of chondrichthyians has been replaced by the theory that placoderms are a group of stem gnathostomes. Taxonomy and phylogenyedit Currently Placodermi are divided into eight recognized orders. There are two further controversial orders. One order is the monotypic Stensioellida, containing the enigmatic Stensioella, and the other order is the equally enigmatic Pseudopetalichthyida. These orders are considered to be basal or primitive groups within Placodermi, though their precise placement within the class remains unsure. Fossils of both are currently known only from the Hunsruck lagerstatten. Cladogramedit The following cladogram shows the interrelationships of placoderms according to Carr et al. (2009):20 Homostius and Pterichthys Diandongpetalichthys |- | |- | | rowspan="2" | |- | |- | | rowspan="2" | |- | |- |Arthrodira | rowspan="2" | |- | |- |Eubrachythoraci | rowspan="2" | |- | |- | | rowspan="2" | |- | |} |- | |- | | rowspan="2" |''Plourdosteus'' |- | |- | | rowspan="2" | |- | |} |- | |} |- | |} |- | |} |- | |} |- | |} |- | |} |- | |} |- | |} |- | |} |- | |} |- | |} See alsoedit * Acanthodii * List of placoderms * Ostracoderm Notesedit # Jump up^ # Jump up^ Entelognathus: The Fish with the First Modern Face # ^ Jump up to:a'' ''b # ^ Jump up to:a'' ''b # ^ Jump up to:a'' ''b # Jump up^ # Jump up^ Long 1983 # Jump up^ Long & Trinajstic 2010 # Jump up^ Long et al. 2008 # Jump up^ Long et al. 2009 # Jump up^ Long 1984 # Jump up^ The first vertebrate sexual organs evolved as an extra pair of legs # Jump up^ Benton, M. J. (2005) Vertebrate Palaeontology, Blackwell, 3rd edition, Figure 3.25 on page 73. # Jump up^ # Jump up^ # Jump up^ Young, G.C. & Goujet, D. & Lelievre, H. (2001) Extraocular muscles and cranial segmentation in primitive gnathostomes – fossil evidence. J. Morphology. 248:304. # Jump up^ Goujet, Daniel & Young, Gavin (2004). Placoderm anatomy and phylogeny: new insights. (in) Arratia, Wilson and Cloutier (eds) Recent Advances in the Origin and Early Radiation of Vertebrates. Verlag Dr. Friedrich Pfeil, Munchen, Germany. # Jump up^ # Jump up^ 1 # Jump up^ Carr RK, Johanson Z and Ritchie A (2009) "The phyllolepid placoderm Cowralepis mclachlani: Insights into the evolution of feeding mechanisms in jawed vertebrates" Journal of morphology, 270 (7): 775–804. doi:10.1002/jmor.10719 Referencesedit * Ahlberg ,P.E., Trinajstic, K., Johanson, Z. & Long, J.A. 2009. Pelvic claspers confirm chondrichthyan-like internal fertilization in arthrodires. Nature 460: 888–889. * Janvier, P. Early Vertebrates Oxford, New York: Oxford University Press, 1998. ISBN 0-19-854047-7 * LONG, J.A. 1983. New bothriolepid fishes from the Late Devonian of Victoria, Australia. Palaeontology 26: 295–320 * Long, J.A. 1984. New phyllolepids from Victoria and the relationships of the group. Proceedings of the Linnean Society of New South Wales 107: 263–308. * Long, J.A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The Johns Hopkins University Press, 1996.ISBN 0-8018-5438-5 * Long, J.A. & Trinajstic, K. 2010. The Late Devonian Gogo Formation Lagerstatte – Exceptional preservation and Diversity in early Vertebrates. Annual Reviews of Earth and Planetary Sciences 38: 665–680 * Long, J.A., Trinajstic, K., Young, G.C. & Senden, T. 2008. Live birth in the Devonian. Nature 453: 650–652. * Long, J.A., Trinajstic, K. & Johanson, Z. 2009. Devonian arthrodire embryos and the origin of internal fertilization in vertebrates. Nature 457: 1124–1127. * Zhu M, X Yu, B Choo, J Wang and L Jia (2012) "An antiarch placoderm shows that pelvic girdles arose at the root of jawed vertebrates" Journal of the Royal Society, Biology Letters, 8 (3): 453–456. External linksedit * Annetta Markussen-Brown, "Devonian Armoured Fish" 2000 * Introduction to the Placodermi Extinct armored fishes with jaws * * Placoderms * Placodermi: Overview * BBC—report on Dunkleosteus terrelli * Category:Life on Earth Category:Prehistoric fish Category:Prehistoric animals Category:Prehistoric life Category:Prehistoric vertebrates Category:Marine life Category:Marine animals Category:Marine organisms Category:Marine vertebrates Category:Animalia Category:Animals with gills Category:Animals with jaws Category:Silurian Category:Devonian Category:Paleozoic Category:Paleozoic animals Category:Vertebrates Category:Egg-laying animals Category:Birth-giving animals